1. Field of the Invention
The present invention relates to a sound input device.
2. Description of the Related Art
In the course of a telephone call, voice recognition or voice recording, it is preferable to collect only the target voice (user's voice). However, the use environment of a sound input device may include sounds except the target voice such as background noise. Thus, there have been developed sound input devices capable of removing noise.
There are known techniques for removing background noise in a use environment including noise. One technique removes noise by using a microphone having high directivity. Another technique removes noise by identifying the direction of arrival of sound waves using a difference in the arrival time of sound waves and subsequent signal processing.
In recent years, electronic devices have been shrinking in size and techniques for downsizing a sound input device are getting more and more important. The above technical ideas are disclosed in JP-A-7-312638, JP-A-9-331377 and JP-A-2001-186241.
FIG. 11 illustrates the frequency response of a differential microphone. The horizontal axis represents a frequency (kHz) and the vertical axis an output sound pressure value (decibel) A numeral 1002 is a graph of a function representing the relationship between the frequency and the output value (decibel) of a differential microphone assumed in case a sound source is at a distance of about 25 mm from the differential microphone (in case the sound source is at the position of a speaker assumed with a close-talking sound input device). A numeral 1004 is a graph of a function representing the relationship between the frequency and the output value (decibel) of a differential microphone assumed in case a sound source is at a distance of about 1000 mm from the differential microphone (noise sufficiently distant from a close-talking sound input device).
While a differential microphone is known to provide an effect to suppress distant noise, the sensitivity of a differential microphone increases in the high frequency range as shown by the numerals 1002 and 1004. Thus, the high-frequency components of the noise from a differential microphone are likely to be emphasized. The high-frequency components of a talker's voice or noise tend to be emphasized to produce unnatural audible effects or nagging sound quality.